1. Field of the Invention
The present invention relates to display-integrated type tablet devices into which display function is integrated for use in personal computers, word processors and the like.
2. Description of the Prior Art
Conventionally, there have been display-integrated type tablet devices in which a display and a tablet are stacked on each other into an integral construction. FIG. 35 schematically illustrates the construction of an electrostatic induction tablet and its driver section to be used for such display-integrated type tablet devices.
An electrostatic induction tablet 101 is formed by securing two glass substrates, one on which column electrodes X.sub.1, X.sub.2, . . . , X.sub.m (hereinafter, an arbitrary column electrode is represented as X) are arranged in parallel and the other on which row electrodes Y.sub.1, Y.sub.2, . . . , Y.sub.n (hereinafter, an arbitrary row electrode is represented as Y) are arranged in parallel, with a spacer (e.g. transparent adhesive) interposed therebetween in such a way that the two groups of electrodes cross each other at right angles and are opposed to each other. Each row electrode Y is connected to a row electrode shift register 102 while each column electrode X is connected to a column electrode shift register 103.
For this construction, the above-mentioned column electrodes X and row electrodes Y are formed approximately transparent by using indium tip oxide (ITO) or the like.
The row electrode shift register 102 and the column electrode shift register 103 are both connected to a timing generation circuit 104. Further, to the timing generation circuit 104 are connected an x-coordinate detection circuit 107 and a y-coordinate detection circuit 108. The x-coordinate detection circuit 107 detects the x-coordinate of the tip of an electronic pen 105 according to both a signal from the timing generation circuit 104 and a signal fed from the electronic pen 105 via an operational amplifier 106, so that it produces an x-coordinate signal representing the x-coordinate. Likewise, the y-coordinate detection circuit 108 puts out a y-coordinate signal representing the y-coordinate of the tip of the electronic pen 105.
The electrostatic induction tablet 101 constructed as above has a light transmittance of approximately 85%. Accordingly, even if the electrostatic induction tablet 101 is stack on an LCD (liquid crystal display), the display screen of the LCD can be viewed through the electrostatic induction tablet 101. This being the case, it is possible to feed any coordinates on the LCD with the electrostatic induction tablet 101 and the electronic pen 105 in such a display-integrated type tablet device constructed by stacking the electrostatic induction tablet 101 on the LCD as described above.
The electrostatic induction tablet 101 constructed as above and its driver section operate in the following way.
First, shift data and a clock signal are transmitted from the timing generation circuit 104 to the column electrode shift register 103. Then scan pulses of a column electrode scan signal such as shown in FIG. 36 are applied in turn from the column electrode shift register 103 to each column electrode X. Next, in similar manner, scan pulses of a row electrode scan signal such as shown in FIG. 36 are applied from the row electrode shift register 102 to each row electrode Y. When this is done, the electronic pen 105 is made close to the surface of the electrostatic induction tablet 101.
As a result, since the tip electrode of the electronic pen 105 (not shown) is coupled with both column electrodes X and row electrodes Y by their respective floating capacities, there develops an induced voltage as shown in FIG. 37(a) at the tip electrode of the electronic pen 105. When this occurs, the tip electrode mentioned above has the operational amplifier 106 connected thereto, wherein the input-side impedance of the tip electrode of the electronic pen 105 is set higher than that on the lead side.
On the basis of the induced voltage thus developed to the tip electrode, tip coordinates of the electronic pen 105 are detected in the manner as described below.
An induced voltage signal of such a waveform as shown in FIG. 37(a) fed from the electronic pen 105 is transformed into a signal of such a waveform as shown in FIG. 37(b) through a low-pass filter and the amplifier, and fed into the x-coordinate detection circuit 107 or the y-coordinate detection circuit 108.
The x-coordinate detection circuit 107 measures the time interval (Ts in FIG. 37(b)) from when the column electrode X.sub.1 has a pulse of the scan signal as shown in FIG. 36 applied thereto from the column electrode shift register 103 to start the scanning of the column electrodes X to when the peak of waveform in the signal from the electronic pen 105 is fed, in accordance with the clock signal from the timing generation circuit 104 and the signal from the electronic pen 105. The x-coordinate detection circuit 107 then generates an x-coordinate signal representing the x-coordinate of the electronic pen 105 depending on the measuring result.
Similarly, the y-coordinate detection circuit 108 measures the time interval from when the scanning of the row electrodes Y is started to when the peak of waveform in the signal from the electronic pen 105 is fed. Then, the y-coordinate detection circuit 108 produces a y-coordinate signal representing the y-coordinate of the tip of the electronic pen 105 depending on the measuring result.
The measurement of time Ts is implemented by counting the number of pulses of the clock signal applied to the row electrode shift register 102 or the column electrode shift register 103.
Besides, it is also possible to calculate the tip coordinates of the electronic pen 105 more accurately in the way as described below. The x-coordinate detection circuit 107 normalizes the peak value of each step in such stepped waveform of the signal from the electronic pen 105 as shown in FIG. 37(a) by maximum peak value. After that, the interval between the x-coordinate of a column electrode X.sub.m1 to which a scan signal is applied while the peak value is presented (i.e. x-coordinate of the column electrode X.sub.m1 closest to the tip of the electronic pen 105, which can be determined from time Ts pertinent to the maximum peak value) and the x-coordinate of a column electrode X.sub.m2 to which a scan signal is applied while the second highest peak value is presented (i.e. x-coordinate of the column electrode X.sub.m2 second closest to the tip of the electronic pen 105) is divided according to the ratio of the aforementioned maximum peak value to the second highest peak value. In this way, the x-coordinate of the tip of the electronic pen 105 positioned between the column electrode X.sub.m1 and the column electrode X.sub.m2 is determined.
In the same manner, the y-coordinate detection circuit 108 determines the y-coordinate of the tip of the electronic pen 105 positioned between a row electrode Y.sub.n1 closest to the tip of the electronic pen 105 and a row electrode Y.sub.n2 second closest thereto, according to the ratio of the maximum peak value to the second highest peak value in the signal from the electronic pen 105.
This way of determination allows the tip coordinates of the electronic pen 105 to be calculated with high accuracy even when the pitch of the column electrodes X or that of the row electrodes Y is rather coarse. In addition, the reason why the peak value of each step of the stepped waveform in a signal from the electronic pen 105 is normalized by maximum peak value is to prevent any error from occurring even when the tip of the electronic pen 105 is out of contact with the surface of the electrostatic induction tablet 101.
As described above, the electrostatic induction tablet 101 is capable of determining the tip coordinates of the electronic pen 105 with high accuracy, in spite of its simple construction, thus lending itself to a variety of applications in small-size computers and the like.
The above-described electrostatic induction tablet 101 and an LCD are stacked together to construct a display-integrated type tablet device which is adapted to display pixels on the LCD corresponding to the tip coordinates of the electronic pen 105 on the electrostatic induction tablet 101. This display-integrated type tablet device presents characters and graphics inputted with the pen on the display screen of the LCD by tracing them on the surface of the electrostatic induction tablet 101 by the tip of the electronic pen 105, thus allowing characters and graphics to be fed with a feeling as if it were writing on paper with a ballpoint pen or some other writing instrument.
However, the above-described display-integrated type tablet device has suffered from some problems as shown below.
A first problem is that it is difficult to view the display screen of the LCD in tracing the surface of the electrostatic induction tablet 101 with the electronic pen 105 while viewing the surface of the display screen of the LCD. More specifically, as described above, column electrodes X and row electrodes Y of the electrostatic induction tablet 101 are formed approximately transparent on transparent substrates of glass, plastic, or the like using tin oxide, indium oxide, or the like. To a disadvantage, light transmittance of electrodes formed as above is approximately 85%, rather low as compared to that of substrates with some mist involved. Also, the electrodes are arranged regularly into a lattice, thereby making the electrodes X.sub.1, X.sub.2, . . . , X.sub.m, Y.sub.1, Y.sub.2, . . . , Y.sub.n unexpectedly noticeable. This phenomenon would be remarkable especially in simplified display-integrated type tablet devices having no back light.
Another problem is that the area where the surface of the LCD's display screen is covered with the electrodes X.sub.1, X.sub.2, . . . , X.sub.m, Y.sub.1, Y.sub.2, . . . , Y.sub.n of the electrostatic induction tablet 101 is relatively large. This makes the LCD's display screen darker and the resulting contrast lower.
Owing to the fact that the LCD and the electrostatic induction tablet 101 are provided separately from each other, there is yet another problem that when integrally assembling the LCD and the electrostatic induction tablet 101 by stacking them on each other, the LCD and the electrostatic induction tablet 101 may be out of position for their correspondence.
In such a case, there occurs some shift between a point on the LCD fed with the pen (i.e. a point specified by the tip of the electronic pen 105) and the point of a pixel displayed on the LCD's display screen by the entry with the pen. This makes it impossible to input characters and graphics with a feeling as if it were writing on paper with a ballpoint pen or some other writing instrument, disadvantageously.
Furthermore, since the LCD and the electrostatic induction tablet 101 each independently constructed are stacked together into one assembly, the resulting display-integrated type tablet device becomes large and heavyweight. Therefore, considerable obstacles will result in offering more compact small-sized computers and word processors that demanders desire. Also, it will account for increase in cost.